Process for producing molten pig iron with melting cyclone

ABSTRACT

A process for producing molten pig iron uses direct reduction of iron ore in a pre-reduction stage followed by a final reduction stage. 
     In the pre-reduction stage iron ore is pre-reduced in a melting cyclone by means of a reducing process gas originating from the final reduction stage. A post-combustion occurs in the reducing process gas in the melting cyclone so that said iron ore in said melting cyclone is at least partly melted. The partly melted iron ore passes downwardly into a metallurgical vessel situated beneath the cyclone in which the final reduction takes place by supply of coal and oxygen, thereby forming a reducing process gas. A partial post-combustion occurs in the reducing process gas in the metallurgical vessel by means of said oxygen supplied thereto. The post-combustion ratio of the gas on exiting the metallurgical vessel is not more than 0.55. The coal is supplied directly into the slag layer so that said partial post-combustion in said metallurgical vessel is at least partly effected in the slag layer.

FIELD OF THE INVENTION

The invention relates to a process for producing molten pig iron bydirect reduction of iron ore comprising a pre-reduction stage and afinal reduction stage, and to apparatus for carrying out the process.

DESCRIPTION OF THE PRIOR ART

Processes of the type described above are known. In one known processthe iron ore is pre-reduced in fluidized state in a reduction shaft. Inanother, the iron ore in the form of pellets is pre-reduced in areduction shaft. In both these processes the temperature in thereduction shaft must be kept low in order to prevent the iron oresoftening and the reduction shaft becoming blocked. Consequently thepre-reduced iron ore is conveyed to a metallurgical vessel in solidstate at a temperature of 600°-900° C. Up to now these processes havenot been used industrially. The problem is that the post-combustion ofthe process gas that occurs in the metallurgical vessel in the finalreduction stage must be high in the metallurgical vessel, that is to sayat least 0.40, in order to generate the heat required in the finalreduction stage at a reasonable coal and oxygen consumption. This heatwhich is released above the melt is only of partial benefit to the melt.If post-combustion is less than 0.40, then a high coal consumptionresults and costly and low volatile coal must be used. In these knownprocesses, on leaving the reduction shaft, the process gas contains muchsensible heat and chemical energy. The sensible heat in the process gasmay be used in different ways. The process gas with the chemical energycontent is called export gas from this point.

In the article "The cyclone converter furnace" by van Langen et al.(Revue de Metallurgie, 90 (1993) No. 3, 363-368), there is disclosed aprocess in which iron ore is pre-reduced in a melting cyclone by meansof a reducing process gas obtained in a final reduction stage. Themelting cyclone is mounted above and in direct communication with ametallurgical vessel in which the final reduction stage takes place.Oxygen and coal are supplied to the melting cyclone. The pre-reducediron ore flows downwardly from the melting cyclone into themetallurgical vessel. In the metallurgical vessel, a slag layer existson top of a bath of pig iron.

EP-A-236802 describes a similar process in which coal is fed into thepig iron bath through bottom tuyeres of the vessel. Hot air at 1200° C.is blown into the vessel, and causes a post-combustion therein so thatthe process gas leaving the vessel has an oxidation degree of 40%. Thehot air at 1200° C. is also blown into the melting cyclone, where asecond post-combustion occurs to an oxidation degree of 80%.

EP-A-237811 describes a process similar to that of EP-A-236802, in whichonly half of the process gas from the metallurgical vessel passes to themelting cyclone, via a passage in which hot air is injected to cause asecond post-combustion so that the gases enter the melting cyclone at2500° C. The molten iron ore passes from the melting cyclone to thevessel via a separate opening.

NL-B-257692 also describes a pre-reduction in a melting cyclone, butdoes not discuss the post-combustion in the vessel.

SUMMARY OF THE INVENTION

An object of the invention is to provide a process for producing moltenpig iron by direct reduction comprising a pre-reduction stage in amelting cyclone and a final reduction stage in a metallurgical vessel,in which, notwithstanding a low post-combustion degree in themetallurgical vessel, a low coal consumption results.

Another object of the invention is to provide a process for producingmolten pig iron by direct reduction, in which it is possible to selectthe degree to which export gas is produced in relation to the use of theexport gas.

According to the invention in one aspect, there is provided a processfor producing molten pig iron by direct reduction of iron ore in apre-reduction stage followed by a final reduction stage, comprising thesteps of

(a) in the pre-reduction stage conveying iron ore into a melting cycloneand pre-reducing it there by means of a reducing process gas originatingfrom the final reduction stage,

(b) effecting a post-combustion in the reducing process gas in themelting cyclone by supplying oxygen thereto so that said iron ore in themelting cyclone is at least partly melted,

(c) permitting the pre-reduced and at least partly melted iron ore topass downwardly from the melting cyclone into a metallurgical vesselsituated beneath it in which the final reduction takes place,

(d) effecting the final reduction in the metallurgical vessel in a slaglayer therein by supplying coal and oxygen to the metallurgical vesseland thereby forming a reducing process gas, and effecting a partialpost-combustion in the reducing process gas in the metallurgical vesselby means of the oxygen supplied thereto, the coal being supplieddirectly into the slag layer,

(e) wherein the post-combustion ratio defined as ##EQU1## in which CO₂,CO, H₂ O and H₂ are the concentrations in percent by volume of thesegases on exiting the metallurgical vessel, is not more than 0.55, and

(f) wherein the partial post-combustion in the metallurgical vessel atleast partly occurs in the slag layer.

The process of the invention produces more export gas with a greaterchemical energy content, the lower the post-combustion ratio is set. Insome cases it is desirable to produce more or less export gas. Thisprocess offers that possibility.

In the invention the coal is supplied directly into the slag layer. Thismeans that the coal enters the slag layer in its solid particulate form,and not via solution in the pig iron bath, as in the method ofEP-A-236802.

The direct injection of the coal into the slag layer, with theconsequence that the first, partial post-combustion occurs at leastpartly in the slag layer, has the consequence that the efficiency oftransfer of heat to the slag and the pig iron bath is high.

Furthermore, a thick slag layer, preferably 1 to 3 m deep may beobtained, in which the partial post-combustion and the reduction of theFeO by the carbon take place. In order to control foaming of the slag,it is desirable that at least some, preferably at least 25%, of the coalis supplied in the form of relatively coarse particles, i.e. particlesof average size 6 mm or more.

Preferably the coal is supplied directly into the slag layer by at leastone of (i) pneumatically transporting finely divided coal by at leastone lance, (ii) pneumatically transporting finely divided coal by meansof at least one side-tuyere of the metallurgical vessel dischargingdirectly into said slag layer, and (iii) dropping coal particles havingan average size of not less than 6 mm into the slag layer. Finelydivided coal is coal having a particle size of less than 6 mm,preferably less than 1 mm.

In the process according to the invention, the coal consumption ispreferably in the range 500 to 1000 kg per tonne of pig iron produced.

In the process, oxygen may be supplied in the form of air, or anothermixture of oxygen and other gas, but preferably the oxygen supplied tothe melting cyclone is injected into the cyclone in the form ofsubstantially pure oxygen. This may be at low temperature, e.g. below100° C. Similarly, preferably the oxygen supplied to the metallurgicalvessel is in the form of substantially pure oxygen and is at atemperature of not more than 100° C.

Preferably the post-combustion ratio of the reducing process gas onexiting from the metallurgical vessel is in the range from 0.20 to 0.55,and more preferably from 0.30 to 0.45. Suitably a post-combustion ratio(as defined above) of the process gas on exit from the melting cycloneis at least 0.60, more preferably at least 0.70, and the coalconsumption is in the range from 600 to 800 kg, more preferably 650 to750 kg, coal per ton of pig iron produced. The process can produce pigiron at a low coal consumption. On leaving the melting cyclone, theprocess gas no longer has so much chemical energy and has a highpost-combustion ratio.

Preferably high volatile coal is used. This is far less costly than lowvolatile coal. It has been found that high volatile coal can be usedwell in the process in accordance with the invention. In the knownprocesses using a reduction shaft it is not possible to use highvolatile coal because of the high post-combustion ratio required withthese processes in the metallurgical vessel.

Preferably the pre-reduction degree (PRD) of the iron ore on leaving themelting cyclone, defined as ##EQU2## is in the range 0.15 to 0.30,wherein 0!_(A) is the oxygen content in mole fraction of the pre-reducediron ore from the melting cyclone and 0!_(B) is the oxygen content inmole fraction of the iron ore supplied to the melting cyclone. Thetemperature of the pre-reduced iron ore on leaving the melting cycloneis desirably in the range 1200° to 1600° C., and preferably the reducingprocess gas is none of cooled, dedusted and reformed between themetallurgical vessel and the melting cyclone. Thus this gas may passdirectly into the melting cyclone from the vessel by the same passage asthe molten part-reduced ore.

Under these process conditions a very low coal consumption may beachieved.

A particular advantage is achieved by a process in which theconcentration of the iron compounds Fe_(x) O_(y) in the slag layer iskept low by supplying the coal to the slag layer at least partly infinely distributed state, i.e. with particle size less than 6 mm. In thefinal reduction of the iron compounds Fe_(x) O_(y) in the slag into pigiron, the coal oxidizes into CO and CO₂. The final reduction occursaccording to a formula of this kind: R=k×A×C. Here R is the reactionrate of the final reduction, k is a constant which however in initialapproximation is inversely proportional to the characteristic, lineardimension of the coal particles, A is the specific surface of the coalparticles, and C is the concentration of the iron compounds Fe_(x) O_(y)in the slag.

Because of the finely divided state of the coal both the constant k andthe specific surface A becomes greater. This results in the finalreduction of the pre-reduced iron compounds Fe_(x) O_(y) coming from themelting cyclone taking place more quickly so that the concentration ofFe_(x) O_(y) in the slag remains lower. The advantage of this is thatthe slag affects the refractory lining of the metallurgical vessel lessquickly. Because of the low wear on the refractory lining, its servicelife is longer.

Preferably the coal is at least partly supplied into the slag in theform of powder coal. This very finely divided state of the coal keepsthe service life of the lining of the metallurgical vessel at a maximum.

In another aspect the invention is embodied in an apparatus for theproduction of molten pig iron by direct reduction of iron ore,comprising

(a) a metallurgical vessel,

(b) supply means for supplying coal directly into a slag layer formed,in operation of the apparatus, above a molten bath of pig iron in themetallurgical vessel,

(c) supply means for supplying oxygen to the metallurgical vessel,

(d) discharge means for discharging molten pig iron and slag from themetallurgical vessel,

(e) a melting cyclone located above and in open connection with themetallurgical vessel so as to form a single reactor therewith, processgas passing in operation from the metallurgical vessel directly into themelting cyclone and at least partially melted pre-reduced iron orepassing from the melting cyclone directly into the metallurgical vessel,

(f) supply means for supplying iron ore into the melting cyclone,

(g) supply means for supplying oxygen into the melting cyclone,

(h) discharge means for discharging process gas in a flow stream fromthe melting cyclone,

(i) a steam-producing boiler in the discharge means for dischargingprocess gas from the melting cyclone for generating steam from sensibleheat of the process gas,

(j) dedusting means downstream of the steam-producing boiler in the flowstream, for dedusting the process gas.

The coal supply means preferably comprises at least one of (i) at leastone lance for pneumatically conveying coal in finely divided form, (ii)at least one side-tuyere of the metallurgical vessel for pneumaticallyconveying coal in finely divided form, and (iii) means forgravitationally dropping coal into the slag layer.

BRIEF INTRODUCTION OF THE DRAWINGS

The invention will be illustrated by description of embodiments, withreference to the drawings, in which:

FIG. 1 is a flow sheet diagrammatically showing a process and apparatusin accordance with the invention.

FIG. 2 shows by way of example the relationship between on the one handthe sensible heat and the chemical energy in the process gas that leavesthe melting cyclone and the coal consumption on the other hand.

FIG. 3 is a graph showing by way of example one operating window of theprocess of the invention.

FIG. 4 is another graph showing another operating window of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a melting cyclone 1 to which iron ore concentrate issupplied with a carrier gas through a supply system 2. At the same timesubstantially pure oxygen is supplied to the melting cyclone 1 via asupply system 3. The term "pure oxygen" is here used as understood inthe steel-making art. Directly beneath the melting cyclone and in openconnection with it is a metallurgical vessel 4. The iron ore ispre-reduced in the melting cyclone 1 and melted by a reducing processgas originating from the metallurgical vessel 4. In this process gas apost-combustion is maintained with the oxygen in the melting cyclone 1.The 15 to 30% pre-reduced and molten iron ore trickles at a temperatureof preferably 1400°-1600° C. down the wall 5 of the melting cyclone 1directly into the metallurgical vessel 4.

In the metallurgical vessel 4 there is during operation a melt 6 of pigiron with a slag layer 7 on top of it. Typically, this slag layer 7 is 2m thick. Substantially pure oxygen is supplied to a lance 12 in themetallurgical vessel 4 by supply system 16 and coal by supply system 9.The pre-reduced iron ore is finally reduced by the coal supplied thussupplied directly into the slag layer 7, whereby a process gascomprising CO₂ and CO is formed that also contains H₂ O and H₂ from thehydrogen originating from the coal. Together with the oxygen supplied tothe metallurgical vessel 4, the process gas is post-combusted in themetallurgical vessel to a post-combustion ratio of preferably 40%maximum. The heat released during this works to the benefit of the slaglayer with a certain heat transfer efficiency (HTE). The process gasflows directly into the melting cyclone 1, is further post-combustedthere as mentioned above, and leaves the melting cyclone 1 with acertain post-combustion ratio. The molten crude iron and the slag aretapped off at 10.

FIG. 1 also indicates that inert gas can be supplied at position 11through the bottom of the metallurgical vessel 4 in order to stir themelt 6.

Together the melting cyclone 1 and the metallurgical vessel 4 form oneentity, that is to say they are directly connected together by anopening through which both the molten iron ore and the process gas pass,without any connecting pipework because the melting cyclone 1 is placeddirectly on top of the metallurgical vessel 4.

FIG. 1 indicates by way of example the supply of oxygen and coal to themetallurgical vessel 4 by means of a central lance 12. Which dischargeswithin, or just above, the slag layer 7. Many variants may be consideredfor this. For the supply of coal, not as lumpy coal but rather in finelydistributed state, the preference goes to one or more lances or tuyeres17, for example through the side wall of the metallurgical vessel 4 bywhich the finely distributed coal, preferably powder coal, is injecteddirectly into the slag layer. This accelerates the final reduction ofthe pre-reduced iron ore in the slag layer 7 so that the refractorylining 13 of the metallurgical vessel 4 at the level of the slag layeris preserved.

As described above, some of the coal may be in lumpy form, i.e. ofsize >6 mm. This may be fed gravitationaly, via suitable openings in thevessel.

The process gas leaves the melting cyclone 1 at a temperature of1200°-1800° C. This sensible heat is converted in a boiler 14 intosteam, from which electricity may be generated. The electrical capacitythus obtained is more than adequate for producing the oxygen required.After boiler 14 the process gas still contains chemical energy by whichelectricity may also be generated.

The process can be run under an elevated pressure of for example 3 barsin the melting cyclone 1 and the metallurgical vessel 4.

FIG. 1 also shows that the process gas is dedusted after the boiler 14in a venturi scrubber 15.

The process gas, which is called export gas after the boiler, stillcontains chemical energy, called export energy from this point, thequantity of which may be selected according to needs by adjusting thecoal consumption of the process beyond the minimum coal consumption thatis needed for the production of pig iron.

FIG. 2 shows by way of example the relationship between the sensibleheat and the chemical energy in the process gas that leaves the meltingcyclone on the one hand and the coal consumption on the other. Theexample of FIG. 2 applies for the case that the post-combustion ratio inthe metallurgical vessel is 25% and that the heat transfer efficiency inthe metallurgical vessel is 80%. The Figure shows that under thesecircumstances in the first instance the sensible heat in the process gasis virtually constant and independent of the coal consumption. Howeverthe chemical energy in the export gas increases with the coalconsumption. The sensible heat in the process gas of about 5 GJ per tonof crude iron which is inevitable can be converted in a boiler intosteam and then into electricity which may be used for the production ofoxygen needed. However the quantity of chemical energy in the export gascan be selected by adjusting the post-combustion ratio. The minimum coalconsumption under the given circumstances is approximately 640 kg perton of crude iron. This Figure shows that in contrast to the knownprocesses using a reduction shaft, the process in accordance with theinvention does not result in a high, undesired quantity of exportenergy, but that the process in accordance with the invention may if sodesired be used with a minimum of coal consumption without excessiveexport energy.

FIG. 3 shows by way of example one operating window of the process inaccordance with the invention. The example of FIG. 3 applies for thecase that the iron ore is pre-reduced in the melting cyclone by 20% andthat the pre-reduced iron ore goes to the metallurgical vessel at atemperature of 1500° C. FIG. 3 takes into account a cooling loss of 500MJ per tonne pig iron and no losses of coal and iron oxide. The exampleof FIG. 3 shows the relationship between the heat transfer efficiencyfrom the metallurgical vessel and the post-combustion ratio in themetallurgical vessel with the coal consumption as parameter. With a lowheat transfer efficiency the temperature of the process gas in themetallurgical vessel is too high; on the other hand there are limits tothe highest value of the heat transfer efficiency of the process gas tothe slag layer and the melt. Where the post-combustion ratio is too highthe process gas in the melting cyclone becomes too lean; there is theninsufficient CO in the process gas for achieving 20% pre-reduction inthe melting cyclone. Where the post-combustion ratio is too low, thecoal consumption becomes too high and too much process gas is produced.For a minimum coal consumption the post-combustion ratio must be high.In the example of FIG. 3 the minimum coal consumption is approximately640 kg per ton of pig iron at a heat transfer efficiency ofapproximately 80%. This means that the post-combustion ratio in themelting cyclone is also high (at least 70%). By optimisation, the coalconsumption could be reduced to 500 kg per ton of crude iron. As shownin FIG. 2, if more export energy is required then the process inaccordance with the invention offers the possibility of generatingexport energy up to approximately 10 GJ per ton of crude iron at a coalconsumption of some 900 kg per ton of crude iron.

FIG. 4 shows another operating window of the process of the invention,in which the post-combustion ratio may range from about 0.25 to 0.55.FIG. 4 takes into account a cooling loss of 1000 MJ per tonne pig ironwhich typically may occur and also losses of coal and iron oxide of 60kg per tonne pig iron each, e.g. as dust. in both FIG. 3 and FIG. 4,medium volatile coal with 32 MJ/kg is used and the coal consumption isin the range 500-1000 kg/tonne pig iron.

What is claimed is:
 1. A process for producing molten pig iron by directreduction of iron ore in a pre-reduction stage followed by a finalreduction stage which forms a reducing process gas, comprising the stepsof(a) conveying iron ore into a melting cyclone and pre-reducing theiron ore in the pre-reduction stage by means of the reducing process gasformed in said final reduction stage; said reducing gas being directlyfed into the melting cyclone from the metallurgical vessel situatedbeneath the melting cyclone, (b) effecting a post-combustion in saidreducing process gas in said melting cyclone by supplying substantiallypure oxygen thereto so that said iron ore in said melting cyclone is atleast partly melted, (c) permitting the pre-reduced and at least partlymelted iron ore to pass downwardly from said melting cyclone into themetallurgical vessel in which said final reduction takes place, and (d)effecting said final reduction in said metallurgical vessel in a slaglayer therein by supplying coal and oxygen to said metallurgical vesseland thereby forming the reducing process gas, and effecting a partialpost-combustion in said reducing process gas in said metallurgicalvessel by means of said substantially pure oxygen supplied thereto, saidcoal being supplied directly into said slag layer, (e) wherein thepost-combustion ratio defined as ##EQU3## in which CO₂, CO, H₂ O and H₂are the concentrations in percent by volume of these gases on exitingsaid metallurgical vessel, is not more than 0.55, and (f) wherein saidpartial post-combustion in said metallurgical vessel at least partlyoccurs in said slag layer.
 2. Process according to claim 1 includingsupplying said coal directly into said slag layer by at least one of (i)pneumatically transporting finely divided coal by at least one lance,(ii) pneumatically transporting finely divided coal by means of at leastone side-tuyere of said metallurgical vessel discharging directly intosaid slag layer, and (iii) dropping coal particles having an averagesize of not less than 6 mm into the slag layer.
 3. Process according toclaim 1 wherein said coal supplied directly into said slag layerincludes at least 25% by weight of particles having a size of not lessthan 6 mm.
 4. Process according to claim 1 wherein the coal consumptionin the process is in the range 500 to 1000 kg per tonne of pig ironproduced.
 5. Process according to claim 1 wherein said coal is suppliedto the slag layer with a carrier gas by means of at least one injectionlance submerged in the slag layer or discharging closely above the slaglayer.
 6. Process according to claim 1 wherein said substantially pureoxygen supplied to said metallurgical vessel is at a temperature of notmore than 100° C.
 7. Process according to claim 1 wherein said postcombustion ratio of the reducing process gas on exiting from saidmetallurgical vessel is in the range from 0.20 to 0.55.
 8. Processaccording to claim 1 wherein said post-combustion ratio of the processgas on exit from the melting cyclone is at least 0.60, and the coalconsumption is in the range from 600 to 800 kg coal per ton of pig ironproduced.
 9. Process according to claim 1 wherein said coal supplied isa high volatile coal.
 10. Process according to claim 1 wherein thepre-reduction degree (PRD) of the iron ore on leaving the meltingcyclone, defined as ##EQU4## is in the range 0.15 to 0.30, wherein0!_(A) is the oxygen content in mole fraction of the pre-reduced ironore from the melting cyclone and 0!_(B) is the oxygen content in molefraction of the iron ore supplied to the melting cyclone.
 11. Processaccording to claim 1 wherein the temperature of the pre-reduced iron oreon leaving the melting cyclone is in the range 1200° to 1600° C. 12.Process according to claim 1 wherein the reducing process gas is none ofcooled, dedusted and reformed between the metallurgical vessel and themelting cyclone.
 13. Process according to claim 1 wherein said coal isat least partly supplied to said slag layer in the form of powder coal.